The invention relates to apparatus and methods for the digital coding of sound signals accompanying a recording made in successive segments at a television image change rate. These signals are stored on the recording carrier of an audiovisual signal diffusion system and in particular comprising a video disc.
It is known to record sound on a video disc by means of samples occupying the time intervals allocated to the beam return trace of the video signal of a colour television image. This recording can relate to a single wide band sound channel or to two multiplexed sound channels.
It is also known that the digital transmission of information has a signal/noise ratio which is generally higher than the analog transmission.
However, the digital recording of information is not free from all problems and defects. Losses or deterioration to bits, particularly in connection with the reading of a recording or structual defects on the recording carrier influence the integrity of the message to be transmitted.
This problem is well known, for example, in teletransmissions or in recording methods intended for information processing. It has been eliminated by various processes, including the use of redundant codes such as multiple error detection codes--simple error correction associated possibly with parity bits.
All these processes involve the use of costly and complex equipment and are therefore unsuitable for the purposes of the invention. In addition, as the time intervals allocated to the beam return trace of the video signal have a limited duration they are relatively unsuited to the use of long codes resulting from a high redundancy. Experimental data also show curves giving the frequency of the faults as a function of the duration which, for the recording carriers used within the scope of the invention make it possible to envisage a better choice of error correction methods.
Finally note should be taken of the physiological characteristics of the ear which permit certain tolerances with regard to the integrity of the signal read and reproduced with respect to the original signal. Thus, it would appear possible to only protect the high weight (most significant) bits of the digital signals. Deterioration of the low weight (least significant) bits would only represent the low amplitude erroneous fluctuations of the sound signal and the human ear could adapt to these.